Preparation of pentaborane(9)



United States Patent 3,460,905 PREPARATION OF PENTABORANE(9) Lawrence J.Edwards, Zelienople, and William V. Hough,

Gibsonia, Pa., assignors to Mine Safety Appliances Company, acorporation of Pennsylvania No Drawing. Original application July 18,1957, Ser. No. 672,573, now Patent No. 3,281,218, dated Oct. 25, 1966.Divided and this application Aug. 1, 1966, Ser.

Int. Cl. C01b 35/00 US. Cl. 23204 32 Claims This application is adivision of Ser. No. 672,573 filed July 18, 1957, which has issued asU.S. Patent No. 3,281,- 218.

This invention relates to boranes, and more particularly this inventionrelates to novel coordination compounds of boranes, methods of makingthese coordination compounds, and methods of making pentaborane(9), B H

Pentaborane(9) has received considerable attention in recent years as areducing agent and as an accelerator for the vulcanization of naturaland synthetic rubbers. Furthermore, a number of recent publications haveshown that pentaborane(9) is a suitable high energy fuel for use inrocket and jet engines. Its usefulness is enhanced by the fact that itis a stable liquid at ordinary temperatures and pressures.

Heretofore the most practical method of preparing pentaborane(9) was bythe pyrolysis of diborane. Various investigators have attempted toincrease the proportion of pentaborane(9) in the mixture of pyrolysisproducts by varying the conditions at which the pyrolysis was performed.The attempts have been only moderately successful because when theconditions are sufficiently severe to form pentaborane (9), thepentaborane(9) itself is consumed by various pyrolytic side reactionsand to avoid this the pentaborane(9) must be removed rapidly from thepyrolysis zone. Under such conditions only a very small proportion ofthe diborane is converted even though the yield of pentaborane(9) basedon the amount of diborane consumed by pyrolysis is relatively high. Ithas become evident that a more convenient method for producingpentaborane(9) in acceptable amounts and yields is required if thiscompound is to receive the potential of uses to which it is entitled.

It is among the objects of this invention to provide a new and usefulmethod of preparing pentaborane(9) in quantitative yields by anon-pyrolytic reaction.

Another object is to provide a new and useful method of preparingpentaborane(9) by the reaction of tetraborane and an adduct oftriborane(7).

Yet another object is to provide a method of preparing pentaborane(9) bypyrolysis of diborane in the presence of an organic substituted hydrideof group VA and VIA elements, an adduct of triborane(7) and an organicsubstituted hydride of group VA and VIA elements, an adduct of borane,EH and an organic substituted hydride of group VA and VIA elements andmixtures thereof.

A further object is to provide new and useful addition compounds oftriborane(7) and an organic substituted hydride of group VA and VIAelements and hydrides of group VA elements.

Another object is to provide a new and useful method of preparing thesenew triborane(7) adducts by the reaction of tetraborane and an organicsubstituted hydride of group VA and VIA elements, and the reaction oftetraborane with the borane adducts of an organic substituted hydride ofgroup VA and VIA elements.

This invention is based on the discovery that tetraborane reacts withcertain coordinating compounds, X, to form a coordination compound, orcomplex XB H which may be considered a triborane(7) addition com-3,460,905 Patented Aug. 12, 1969 pound or adduct. This triborane(7)adduct may react further under suitable conditions with tetraborane toquantitatively yield pentaborane(9). It has also been discovered thatwhen diborane is pyrolyzed under suitable conditions in the presence ofsuch coordinating compound, X, adducts of triborane(7), XB3H7, adductsof borane, XBH or mixtures thereof the pyrolysis products contain anexceptionally high proportion of pentaborane (9), and an exceptionallylow amount of solid BH polymers. We have discovered that coordinatingcompounds which function in this invention are those compounds which canbe considered as organic substituted hydrides of group VA and VIAelements which are basic in character, that is, compounds in which thegroup VA and VIA element is capable of donating an electron pair.Amines, ethers, thioethers, phosphines, and arsines are included in thisclassification.

This invention is more fully described hereinafter and and the noveltythereof will be particularly pointed out and distinctly claimed.

According to this invention pentaborane(9) is produced from tetraboraneand a coordinating compound by a two step reaction. A triborane(7)adduct is first produced according to Equation 1 The triborane(7) adductreacts with additional tetraborane to give 2 mols of pentaborane(9) foreach additional 4 mols of tetraborane; this step may be represented byEquation 2 In the overall process, then, 2 mols of pentaborane(9) areproduced for each 5 mols of tetraborane consumed. A mixture of thereactants and products of either reaction (1) or (2) contains thereactants of the other reaction, so that both reactions may proceedsimultaneously. The rate at which reaction (1) proceeds in the forwarddirection appears to be determined by the base strength of thecoordinating reactant. For example, with strong bases the reactions goesinstantly to completion, While with very Weak bases the reaction is veryslow and there is a low equilibrium concentration of the triborane(7)adduct. Reaction (2) proceeds at a relatively slow rate and does notappear to be reversible to any significant extent. The rate is dependentin the usual manner on reactant concentration and on the strength of thebase component of the triborane(7) adduct, it appears that the strongerthe base the slower the rate of reaction. The base strength of thecoordinating compound or other terms connoting basicity as used hereinare relative terms in accord with common usage. It refers to itscapability to coordinate with B H- and it is believed that it isafiected both by intramolecular electronic effects and steric eifectscaused by molecular structures. There is a continuous spectrum of basestrength from very strong to very weak in this group of organicsubstituted hydrides of elements of group V-A and VIA, and the processvariations caused by variation in base strength are for conveniencehereinafter discussed in terms of an arbitrary division into verystrong, strong, weak, and very weak base strength.

The reactions of strong bases, e.g. tetrahydrofuran, with tetraboraneclearly show the two step reaction to form pentaborane (9). Tetraboraneand tetrahydrofuran react rapidly to form tetrahydrofuran triborane (7)and diborane. Tetraborane (6.48 millimoles) and tetrahydrofuran (6.58millimoles) were frozen in a reaction bulb connected to a vacuum system.The reaction bulb was allowed to warm and when the products became fluidan immediate and vigorous reaction occurred from which diborane wasevolved. The diborane evolution appeared to stop in about five minutesindicating the completion of the reaction. The product was a viscous,colorless liquid at room temperature and was found to be a mixture oftetrahydrofuran triborane (7), C H OB H and absorbed diborane. Theabsorbed diborane was re moved from this product by subjecting themixture to a vacuum at room temperature. The residue remaining in theflask was a white solid, containing 3 milliatoms of boron and 6.98milliatoms of hydrogen for each 1.01 millimoles of tetrahydrofuran ascompared to the theoretical 3 milliatoms of boron and 7 milliatoms ofhydrogen for each millimole of tetrahydrofuran for the pure compound,tetrahydrofuran triborane (7). The reaction of tetraborane withtetrahydrofuran triborane (7) (to produce pentaborane (9)) is quiteslow. For example, when 6.7 millimoles of tetraborane were contactedwith 1.99 millimoles of tetrahydrofuran triborane (7) at roomtemperature for a period of 5 hours it was found that 43% of thetetraborane had reacted. For each 2 /2 moles of tetraborane consumed0.98 mole of pentaborane (9) was produced. The amount of tetrahydrofurantriborane (7) recovered was equal to the amount charged. As thetetraihydrofuran triborane (7) is regenerated as fast as it is consumedthe overall reaction may be regarded as catalyzed by the triborane (7)adduct:

catalyst The catalyst, of course, does not have to be separatelyprepared and isolated, but can be formed in situ from tetrahydrofuranand tetraborane. Thus when 9.10 mmoles of tetraborane and 0.66 mmole oftetrahydrofuran were frozen into a reaction bulb, warmed to roomtemperature, and maintained at room temperature for a period of 5 hours,2.41 mmoles of tetraborane were consumed. Of this, 0.62 mmole wereconsumed to form tetrahydrofuran triborane, and 1.75 mmoles wereconsumed by the reaction forming pentaborane (9). A quantitative yieldof pentaborane (9) was recovered by fractional distillation under vacuumfrom the reaction mixture. The recovery of tetrahydrofuran triborane(7)was about 95%. If the reactants are left in contact for longer periods,a higher proportion of the tetraborane reacts. Thus, when 9.02millimoles of tetraborane were reacted with .66 millimole oftetrahydrofuran in the same manner for a period of 16 hours, 5.33millimoles of the tetraborane were consumed by the reaction (4.67 mmolesconsumed in pentaborane(9) forming reaction). The yields ofpentaborane(9) and tetrahydrofuran triborane(7) were over 95% based onEquations 1 and 2.

The reactions of strong bases with tetraborane are characterized by theformation of relatively stable triborane(7) adducts. The rate at whichthe triborane(7) adduct forms from tetraborane and the strong base isseveral orders of magnitude greater than the rate at which thetriborane(7) adduct reacts with tetraborane to give pentahorane(9)Tetraborane reacts with weak bases to form a triborane(7) adduct at arate much slower than with strong bases. The group of weak bases ischaracterized by a substantial equilibrium concentration of triborane(7)adduct; i.e., when the rate of formation of the triborane(7) adduct byreaction (1) is substantially equal to the rate the triborane(7) adductis consumed by reaction (2) there is an equilibrium mixture of the baseand substantial amount of triborane(7) adduct. The actual equilibriumconcentration can be adjusted to a certain extent by the diborane andtetraborane concentrations, but is primarily a function of the basicityof the base. The triborane(7) adducts with weak bases are normallyliquids at room temperature and are relatively unstable.

The following examples of the reactions of ethyl ether illustrate theweak base reactions. Tetraborane (9.50 millimoles) and ethyl ether(14.50 millimoles) were frozen into a reaction bulb attached to a vacuumsystem. After 5 hours contact at room temperature 4.04 mmoles oftetraborane had been consumed. The gaseous products,

hydrogen and diborane, were removed, leaving a liquid mixture ofpentaborane(9), ethyl ether, ethyl ether tri borane(7), and someabsorbed diborane. This mixture was fractionated under vacuum at lowtemperature (about 10 C.); ethyl ether and 1.56 mmoles of pentaborane(9)were removed and 0.85 mmole of ethyl ether triborane(7) residue remainedin the bulb. The ethyl ether triborane(7) adduct is the least volatilecomponent in the mixture, exerting a pressure of 3 mm. at 0 C., and is aliquid at room temperature. The compound is relatively unstable at roomtemperature; it almost completely decomposes over a period of severaldays. The product contained about 1.02 millimoles of ether to 3milliatoms of boron to 7.02 milliatoms of hydrogen compared totheoretical ratio of 1:3:7. In another reaction 6.98 mmolesof=tetraborane and 15 mmoles of ethyl ether were contacted in the samemanner for a period of 6 hours. 49% (3.43 mmoles) of the chargedtetraborane was consumed, yielding 1.08 mmoles of pentahorane(9) and0.70 mmole of ethyl ether triborane(7). With longer reaction periodsmore tetraborane is consumed to produce pentahorane(9); however, theconcentration of ethyl ether triborane(7) does not appear to increase.Since the equilibrium amount oftriborane(7) adduct is continuallyregenerated the overall reaction may be regarded as catalyzed by themixture:

513-11110 213 11 -I-- 5BZHB H2 The reaction of very weak bases withtetraborane to form the triborane(7) adduct is even slower than withweak bases, thus the equilibrium concentration of the triborane(7)adduct is very low, i.e. a trace amount. For example, when 2.45 mmolesof tetraborane were contacted with 2.52 mmoles of diphenylmethylaminefor 40 hours at room temperature 1.75 mmoles of tetraborane wasconsumed. A yield of pentahorane(9) based on equations 1) and (2) wasrecovered. The triborane(7) adduct could not be found by usual chemicalanalyses, but the presence of trace amounts was indicated by infraredspectrum analyses. Inasmuch as the equilibrium amount of triboraneadduct is only a trace amount the overall reaction may be considered asbeing catalyzed by the base:

2B5H9 1 2 5 H2 The reactions of tetraborane with very strong bases iscomplicated in that the by-product diborane reacts with the base to forma borane adduct,

2X+B H 2XBH which results in the overall reaction (3) B H +2X+XB H +XBHTetraborane reacts also with the borane adduct to form additionaltriborane adduct according to B4H10+XBH3'9 (B3H7+B2H6 in addition to thepentahorane(9) producing reaction,

4B4H10+XB Hq 2B5H +9/2B2H +X+H2 Reaction (3), which produces thetriborane(7) adduct and the borane adduct, is extremely rapid andproceeds instantaneously even at low temperatures. Reaction (4) is quiteslow, but with any given very strong base is more rapid than reaction(5). That is, the concentration of XB3H'] is higher than theconcentration of XBH at equilibrium conditions. Thus when tetraborane iscontacted with a very strong base, pentahorane(9) is produced and anequilibrium mixture of the triborane(7) adduct and the borane adduct ismaintained. As this equilibrium mixture is maintained regardless of theamount of tetraborane consumed by the overall reaction topentaborane(9), the reaction may be regarded as catalyzed by the mixtureaccording to:

The reactions of tetraborane with very strong bases are hereinafterillustrated by reactions of trimethylamine. Tetraborane, 1.77 mmoles,containing 0.24 mmole diborane impurity, was frozen in a reaction bulbcontaining 4.00 mmoles of frozen trimethylamine. The reaction bulb wasallowed to warm, and as the reactants melted, an immediate vigorousreaction occurred producing an equimolar mixture of crystals oftrimethylamine triborane(7) and trimethylamine borane. There were novolatile products formed. The theoretical boron content of the productmixture, taking into account the diborane impurity charged, was 21.17mat./ g. (milligram atoms per gram of sample) the actual boron contentwas determined to be 21.2 mat./g. Infra-red spectrum analysis showedthat the product contained trimethylamine triborane(7).

Tetraborane, 4.56 mmoles, and trimethylamine borane, 3.96 mmoles, werecharged into a reaction bulb maintained at room temperature. After /2hour, about of the tetraborane had reacted. After 40 hours 90% of thetetraborane had reacted, of which 25% was consumed by the reactionproducing pentaborane(9). The solid product was approximately 67%trimethylamine triborane (7) and 33% trimethylamine borane. With highertetraborane concentration the reaction equilibrium is shifted to givelarger amounts of triborane (7 adduct.

The trimethylamine triborane(7) may be formed in situ by the reaction oftetraborane and trimethylamine. Thus when 5.06 mmoles of tetraborane wasfrozen onto 1.03 mmoles of frozen trimethylamine and allowed to warm toroom temperature a vigorous reaction occurred producing equimolaramounts 0 trimethylamine triborane(7) and trimethylamine borane. Thismixture was then allowed to stand at room temperature for 48 hours andover 90% of the borane adduct was converted to trimethylaminetriborane(7) and about of the tetraborane was consumed to give 0.39mmole of pentaborane(9). With longer reaction periods, or with additionof more tetraborane, more tetraborane is consumed to form pentaborane(9)with no further change in the equilibrium concentration of triborane(7)adduct.

It has been found that completely organic substituted hydrides ofelements from group V-A and VI-A, which are bases, are operable in theinvention. The hydrides of group V-A may be only partially substituted;however, only completely substituted hydrides of group VI-A elements aredesirable, because the R X-H type compounds are apt to be involved inundesirable side reactions. The coordination compound (or adduct) withtriborane(7) is formed by a coordination bond involving an availableelectron pair of the group V -A and VI-A element. Thus there is a widerange of base strengths possible as the coordinating electron pair canbe made more or less accessible by blocking with large molecularstructures, i.e. steric hindrance. For example the amines, phosphines,and arsines range from very strong to very weak bases. Lower alkylamines (e.g. primary, secondary and tertiary methyl and ethyl amines,propyldimethylamine, butyldimethylamine), and secondary and tertiarycyclic amines (e.g. pyridine, lutidine, picoline, collidine,pyrrolidine, pyrazine and pyrrole) are very strong bases. Thephosphorous analogues of these amines, e.g. trimethyl phosphines, arealso very strong bases. However, amines or phosphines containing threelarger alkyl groups or an aryl group (e.g. phenyldimethylamine ortri-n-propylamine) are weak bases; and amines or phosphines containingtwo or more aryl groups (e.g. diphenylmethylamine) are very weak bases.Substituted alkyl, aryl, or alkyl aryl amines are operable in the samemanner benzyldimethylamine is a reasonably strong base whiledibenzylmethylamine is a very weak base. The base strength of phosphineanalogues of the amines is comparable to the base strength of theamines. The base strength of the arsines is less than the correspondingamine analogue.

The ethers and thioethers vary from strong bases to very weak bases. Thecyclic ethers (e.g. tetrahydrofuran, furan, tetrahyd-ropyran,chlorofurfuran, tetrahydrofurfurylchloride, dimethylfuran, and diethylacetal) and dimethl ether are strong bases. Lower alkyl ethers, otherthan dimethyl ether, (e.g. diethyl ether, di-n-propyl ether, and propylmethyl ether) and lower alkyl aryl ethers (e.g. anisole) are weak bases.Diaryl ethers, such as diphenyl ether are very weak bases. Thioethersare somewhat weaker than the oxygen analogues, for example, thiophene isa weak base While furan is a stronge base.

The reactions proceed as described it the organic substituted groupsthemselves contain substituted groups provided that the lattersubstituted group is not reactive in the environment of the reaction.For example, if the organic substituted group contains a carbonyl group,the carbonyl group 'will react with the tetraborane or diborane.

It is preferred to carry out the reaction to produce pentaborane(9) fromtetraborane at or slightly above room temperature, although it proceedsover a wide temperature range. At low temperatures, eg 0 C., thepentaborane(9) producing reaction is slow and there is no practicaladvantage to operating under such conditions. At higher temperatures,e.g. 50-60 C., the tetraborane, present in relatively highconcentrations, decomposes to an appreciable extent resulting in lowyields of pentaborane (9) It has been discovered that when diborane ispyrolyzed in the presence of the above described bases there isrecovered a higher yield of pentaborane(9) and a lower loss to solid BHpolymers than when diborane is pyrolyzed alone under the same conditions(of temperature, pressure and time). It is believed that these desirableeffects result from the reaction of tetraborane, the early pyrolysisproduct of diborane, with the base to give a quantitative yield ofpentaborane(9). An alternative plausible mechanism is that triborane(7)is an early formed pyrolysis product and reacts directly with the baseand tetraborane need not exist per so; this is in conformation with thehypothesis that tetraborane is formed from an initial unstable pyrolysisproduct, triborane(7). It is known, however, that tetraborane is anearly product from the pyrolysis of diborane, and that the otherpyrolysis products, including the desirable pentaborane(9) andundesirable BH polymers, result from additional pyrolysis reactionsinvolving tetraborane. These pyrolysis reactions occur simultaneouslyand produce a complete spectrum of higher boranes and solidboron-hydrogen polymers. Heretofore, it has been possible to control thepyrolysis reaction to some extent by varying conditions so that somewhathigher or lower yields of specific higher boranes were obtained.

When diborane is pyrolyzed in the presence of a base as describedherein, however, a proportion of the initially produced tetraboranereacts with the base to give quantitative yields of pentaborane(9)according to and only a portion of the tetraborane is consumed by thepyrolysis reactions that give much lower yields of pentaborane(9). Undercertain conditions extremely high yields, i.e. about of pentaborane(9)can be obtained with a relatively high conversion of diborane (20-25%);and at other conditions essentially all the diborane can be convertedwith at least 60-70% yields of pentaborane(9). Furthermore, whendiborane is pyrolyzed under suitable conditions in the presence of thebase the yield of pentaborane(9) is increased over that obtained frompyrolysis in the absence of the base as some portion of the tetraboranereacts to quantitatively produce pentaborane(9).

The base, and equilibrium mixtures of base, triborane(7) adduct andborane 'adduct, have been found to be effective catalysts to increasethe yield of pentaborane(9) over a wide range of concentration. It isgenerally preferred to use between .02 and .50 mole of catalyst for eachmole of diborane. With less than about .02 mole of catalyst (per mole ofdiborane) the reaction is not significantly improved over uncatalyzedpyrolysis reactions. The yield of pentaborane increases with increasingamount of catalyst; however, there is only a slight advantage realizedby increasing the amount of catalyst over 0.5 mole per mole of diborane.This is illustrated by the results set forth in Tables 1, 2 and 3.

TABLE 1.IYROLYSIS OF DIBORANE WITH (CHiliNBH; CATALYST [Reaction time 16hours; teziiiperature 80 C.; initial BzHa pressure,

mm. Hg]

Percent Catalyst Concentration Moles Percent BqHe K Yield 2 ofCatalyst/mole BzHa Converted Pentaborane(0) See footnotes at end ofTable 2.

TABLE 2.PYROLYSIS OF DIBORANE WITH (CHs)aNBH CATALYST [Reaction time 16hours; temperature 80 0.; initial BgHe pressure, 1,200

1 Throughout the specification percent diborane converted means theactual diborane consumed by all reactions, i.e. the BzHe charged minusthe diborane recovered.

2 Percent yield is the B recovered as B H /total B consumed by allreactions.

TABLE 3.PYROLYSIS OF DIBORANE WITH (CHMNBII;

CATALYST [Reaction time 2% hours; temperature 100 0.; initial BzHspressure 750 p.s.i.g.l

Catalyst Concentration Moles Percent BgHe Percent Yield of Catalyst/moleBzHe Converted Pentaborane (9) In all the catalyzed pyrolysis reactionsthe base was charged to the closed reactor, the diborane wassubsequently added, and the reactor was then brought to the desiredtemperature. After a suitable reaction period, the volatile productswere separated by fractional distillation. When a strong base, such astrimethylamine, was used a portion of the diborane reacted immediatelyon contact to form the borane adduct. The borane adduct is considered asthe catalyst when using a strong base as there is only a small amount oftriborane(7) adduct in equilibrium when a large excess of diborane ispresent.

Those reaction conditions that affect the pyrolysis reaction, i.e.temperature, pressure and reaction time, also affect the yield ofpentaborane(9) when the catalyst is used. The pentaborane(9) producingreaction via the triborane(7) adduct proceeds regularly so that thelonger the reaction time the higher the yield. However, when thepyrolysis conditions are too severe (i.e. temperatures above about 100C.) pentaborane(9) is apparently consumed by further undesirablepyrolysis reactions so that under such severe conditions moderatereaction times of about TABLE 4.PYROLYSIS OF DIBORANE WITH (OH3)3NBH3CATALYST [Initial BzHs pressure 360 nun; temperature 125 C.; 0.3 molescatalyst/mole BzHfi] Percent BzHo Percent Yield Reaction Time (hours)Converted Pentaborane As reaction time is increased the amount ofdiborane converted increases; however, the yield of pentaborane(9)decreases. This phenomenon is characteristic of pyrolysis reactions, andwould indicate that under such conditions a higher proportion oftetraborane is consumed by pyrolysis reactions than at more moderateconditions. The phenomenon also occurs when diborane under pressure ispyrolyzed if the temperature is sufliciently high. Thus when 55 mmolesof B H at 750 p.s.i.g. was contact with 2 mmoles of (CH N at 100 C. for2 /2 hours, 56% of the diborane was converted and the yield of pentaborane(9) was 22%. In a 20 /2 hour reaction under the same conditions83% of the diborane was converted, but the yield of pentaborane(9) wasonly 7%. At more moderate temperature conditions, below about 100 C.,the excessive loss of pentaborane(9) product by pyrolysis does notoccur, and longer reactions result in increased conversion of diboranewithout decrease of pentaborane(9) yield, as is shown in Table 5.

TABLE 5.-PYROLYSIS OF DIBORANE WITH (CHa)aNBH CATALYST [Temperature 600.; initial 13 2H5 pressure 750 p.s.i.g.; 8 moles catalyst/54 molesB2H6] Percent B2He Percent yield Reaction Time (hours) Converted B 119The reaction to produce pentaborane(9) proceeds over a wide temperaturerange, about 40 to 160 C., and at all pressures, i.e. from severalmillimeters of mercury to several hundred atmospheres. The rate at whichdiborane is converted or consumed increases with increasing temperature,e.g. after 16 hours at 600 mm. B H pressure the conversion isapproximately 50% at C., and is approximately 80% at 100 C.; and withincreasing pressure, e.g. after 16 hours at 100 C. the conversion isabout 60% at 360 mm. pressure, and is approximately 80% at 600 mm.pressure. It is generally preferred to operate at high pressures, aboveabout 200 p.s.i.g., and at moderate temperatures from about 50 to C.Under these conditions the rate of conversion is relatively fast andbecause of the moderate temperature the losses of pentaborane(9) productby pyrolysis reactions are slight. At low pressures (about atmospheric)in order to get a high rate of conversion it is preferred to operate athigher temperatures, from about to C., and as set forth above,relatively short reaction periods are required to prevent losses ofpentaborane(9).

The pyrolysis reactions are catalyzed by other organic substitutedhydrides of elements from group V-A and VI-A at suitable conditions inthe same manner as described above for the very strong base,trimethylamine. For example, when 50 mmoles of diborane, at 500 p.s.i.g.and 16.0 mmoles of ethyl ether, a weak base, were contacted at 40 C. for88 hours, 36% of the diborane was consumed. The yield of pentaborane (9)was 63.6% and 15.6% yield of tetraborane was also recovered. Similarly,when 16 mmoles of ethyl ether and 51 mmoles of diborane were contactedat 40 C. for 65 hours, 50% of the diborane was consumed and the yield ofpentaborane (9) was 71%. Similarly when 253 mmoles of B H at 400 mm.pressure, and 6.7 mmoles of tripropylamine, were contacted for 1.5 hoursat 125 C., the conversion of diborane was 33% and the yield ofpentaborane (9) was 28%.

The pyrolysis of diborane in the presence of the base catalyst isparticularly suited to continuous operation. Diborane under pressure iscontinuously fed to a reactor containing the catalyst, which may be apacked bed for solid catalyst such as trimethylamine borane, or a liquidcontacting column for a liquid catalyst such as ethyl ether or phenylether. The products are continuously removed and separated from theunreacted diborane by fractional condensation, and the unreacteddiborane is returned for further reaction.

The triborane (7) adducts are valuable compounds for use as reducingagents and preparing metal triborohydrides as well as for thepreparation of pentaborane (9) and it is therefore desirable to be ableto carry out the reactions of tetraborane and the base so that thetriborane (7) adduct can be recovered in good yields. From the previousdescription of the role of the triborane (7 adduct in the preparation ofpentaborane (9) it is apparent that the reactions of tetraborane thatproduce the adduct must be promoted and the reaction of tetraborane withthe adduct must be retarded.

When using strong bases, such as cyclic ethers, or very strong bases,such as lower alkyl amines or phosphines, or cyclic amines orphosphines, quantitative yields of the triborane (7) adduct are easilyobtained as the reaction, producing the adduct is extremely fast and thereaction consuming the adduct is very slow. Thus when 6.48 mmoles oftetrahydrofuran (a strong base) and 6.58 mmoles of tetraborane werefrozen into a reaction bulb and permitted to warm, an immediate vigorousreaction occurred yielding diborane and a liquid product. The liquidproduct was a mixture of absorbed diborane and tetrahydrofuran triborane(7). The absorbed diborane was removed by vacuum desorption at roomtemperature leaving a white solid which analyzed as tetrahydrofurantriborane (7). The compound melts at 39 C., and has a decompositionpressure at room temperature of less than 1 mm. of Hg. At temperaturesabove the melting point the compound is relatively unstable. As thereaction is rapid and essentially quantitative there is no necessity touse an excess of either reactant; however, if desired an excess ofeither reactant can be used if the excess reactant is promptly removedfrom the triborane (7) product. When a large molar excess oftetraborane, e.g. 600%, is used the initial reaction to formtetrahydrofuran triborane (7) proceeds in the same manner as whenstoichiometric amounts are used, and the excess tetraborane can bedistilled from the mixture if it is removed before about -30 minutesafter contact. If the excess tetraborane is allowed to remain in thereaction mixture for extended periods (several hours) it is partiallyconsumed by pentaborane (9) producing reaction according to Equation 2..

Similarly the initial triborane (7) forming reaction of tetraborane witha very strong base is extremely rapid and goes to completion so that itis preferred to use stoichiometric amounts of reactants according to Forexample, 2.00 mmoles of tetraborane and 4.00 mmoles of trimethylaminewere frozen into a reaction bulb, and then permitted to warm. As thereactants melted there was an immediate vigorous reaction which produceda mixture of crystals of trimethylamine triborane (7) and trimethylamineborane. The triborane (7) adducts can be separated from the boraneadducts by a variety of methods. A convenient rapid method is to destroythe borane adduct by hydrolysis with a dilute acid which does not reactwith the triborane (7) adduct. For example, a portion of the aboveprepared mixture of trimethylamine triborane (7) and trimethylamineborane was contacted with a dilute solution of hydrochloric acid at roomtemperature. The borane adduct was readily hydrolyzed and thetrimethylamine triborane (7) did not dissolve or appear to react. Thesolution was filtered off and the residue was washed with water anddried by vacuum treatment at about 30 C. The dried residue wastrimethylamine triborane (7) of 98% purity. The triborane (7) adductshave also been separated from mixtures in good yields by fractionalsublimation under vacuum at about 6080 C. and by fractionalcrystallization from methanol or isopropanol solutions. The theoreticalcontent of trimethylamine triborane (7) is: B, 30.4 mats./g.; nitrogen,10.1 mats./g.; C, 30.4 mats./g.; hydrogen by hydrolysis, 81.3 mmoles/g;and total hydrogen 163 mats./ g. The actual elemental content oftrimethylamine triborane (7) prepared by the above methods andrecrystallized from methanol was found to be: B, 30.7 mats./g.;nitrogen, 10.0 mats/g; carbon, 30.3 mats./ g.; hydrogen by hydrolysis79.5 mmoles/g; and total hydrogen, 167 mats./g.

As was disclosed above, tetraborane reacts further with the boraneadduct to form triborane (7) adduct, and reacts with the triborane (7)adduct to form pentaborane (9), giving an equilibrium mixture of theborane and triborane (7) adducts, which is rich in the triborane (7)adduct if the tetraborane concentration is high. As these reactions arevery slow, excess tetraborane can be contacted with the very strongbase, and the excess recovered by immediate separation (as bydistillation) from the initial reaction products. The excess tetraboranecan be left in contact with the initial reaction products for extendedperiods (2040 hours) and the triborane (7) adduct rich mixture isrecovered; under these conditions some tetraborane is consumed toproduce pentaborane (9), which is a loss when the object is to preparetriborane (7) adduct. For example, when 1.75 mmoles of tetraborane and1.00 mmoles of trirnethylamine were contacted for 40 hours at roomtemperature the recovered adduct mixture was over trimethylaminetriborane (7) and about 20% of the tetraborane charged was consumed bythe pentaborane 9) reaction.

Very strong bases appear to react slowly with triborane (7) adducts togive unidentified degradation products, so that if the very strong baseis present in excess it should be removed immediately after the initialreaction.

The initial reaction of strong and very strong bases is rapid andproceeds satisfactorily at any temperature at which one of the reactantsis fluid. It is preferred to carry out the secondary reaction oftetraborane with the borane adduct of a very strong base at about roomtemperature. At low temperatures, e.g. 0 C., the reaction is slow and atelevated temperatures, e.g. 50 C., the losses of tetraborane by thermaldecomposition are large over the extended reaction periods required. Themethod of bringing the reactants into contact are not critical. It isconvenient in small scale preparations to free both reactants in areaction bulb and then permit the reactants to warm; the reaction thenproceeds when the reactants become fluid. This procedure is cumbersomefor larger scale preparation and for continuous processes, and in thesecases it is preferred to mix the liquid reactants, mix a vapor andliquid reactant stream (above the boiling point of tetraborane, 16 C.,gaseous tetraborane can be reacted with a liquid base and below theboiling point of tetraborane a gaseous base can be reacted with theliquid tetraborane), or mix two gaseous reactants (as with tetraboraneand methylamine or methyl ether). It is possible to carry out thereaction in the presence of an inert liquid, such as a hydrocarbon, e.g.hexane. It is particularly preferred to use an inert liquid to moderateand aid in control of those reactions in which both reactants aregaseous. The inert diluent is also of particular use in those reactionswhich produce a solid triborane(7) compound, as it facilitates thehandling of the solid product, i.e. as a slurry. The inert liquid can befiltered or centrifuged or evaporated from the triborane(7) coordinationcompounds.

The initial reaction with partially substituted hydrides of group V-Aelements proceed in the same manner, but the borane adducts of suchcompounds are relatively less stable and may thermally decompose. Forexample, when monomethylamine and tetraborane are contacted at ambienttemperatures they react according to The borane, unstable at ambienttemperature, decomposes according to resulting in a product which is amixture of methylamine triborane(7) and methylaminoborane. Thetriborane(7) compound can be separated from the aminoborane by the sametype procedures as used for the separation from amine boranes.

Quantitative yields of triborane(7) adducts of Weak bases are difiicultto obtain because the rate at which the triborane(7) adduct is formed isin the same order of magnitude as the rate at which it is consumed bythe pentaborane(9) reaction. Thus when 8.92 mmoles of tetraborane and15.0 mmoles of ethyl ether were contacted at room temperature for hours,1.79 mmoles of tetraborane were consumed to yield .89 mmole ofpentaborane and 1.39 mmoles of tetraborane were consumed to yield ethylether triborane(7) (about 10% concentration in ethyl ether). The excessethyl ether and tetraborane were distilled from the reaction mixtureunder vacuum at room temperature leaving a liquid residue of ethyl ethertriborane(7). Similarly, 2.28 mmoles of tri-n-propylamine was contactedwith 2.49 mmoles of tetraborane at room temperature for 16 hours and 66%of the tetraborane was consumed in forming triborane(7) adduct and 7%was consumed to form penta'borane(9).

The complexes of this invention may be conveniently produced in ametathetic reaction from a complex formed from a coordinating compoundof a Weaker base strength than the desired complex. For example,trimethylamine triborane(7) has been produced by adding trimethylamineto tetrahydrofuran triborane dissolved in tetrahydrofuran or anothersuitable solvent having a base strength no greater than that oftetrahydrofuran. The tetrahydrofuran and solvent was removed by vacuumcondensation leaving the pure solid, trimethylamine triborane(7).Ammonia triborane(7) has been produced by condensing a stoichiometricquantity of ammonia in an ethyl ether or tetrahydrofuran solution oftetrahydrofuran triborane(7). The liquid was removed under vacuum atroom temperature and the white solid ammonia triborane(7) was recoveredby sublimation under vacuum at 40 to C. In like manner trimethylphosphine triborane(7) has been prepared by the addition of trimethylphosphine to tetrahydrofuran triborane(7) or ethyl ether triborane(7)dissolved in ethyl ether.

An alternate method of formation of complexes of ammonia or aminesinvolves the reaction of ammonium halide or an organic substitutedammonium halide with sodium triborohydride, NaB H according to theequation The reactants are agitated in ethyl ether because of therelatively low solubility of the reactants over a period of time at roomtemperature. The insoluble solids are then filtered off and the solventremoved from the filtrate depositing a white solid which was sublimed at40 to 50 C.

The triborane(7) coordination compounds have been identified andcharacterized by a number of independent methods. The empiricalcomposition is determined by elemental analyses. The hydrogen (termedhydrogen by hydrolysis) from the triborane(7) portion of the molecule isdetermined by hydrolysis according to the equation Since the compoundsare quite resistant to hydrolysis, it is necessary to hydrolyze thesample in concentrated hydrochloric acid at about C. for several days.The boron content is determined by the usual procedure of titrating thehydrolyzed sample in the presence of mannitol. The total carbon andhydrogen content is obtained by conventional microcombustion procedures.The group V-A and VI-A elements, except oxygen are determined byconventional procedures, e.g. nitrogen is determined by Kjeldahlanalysis. Ethers were determined by separating, purifying and weighingthe ether layers which form an immiscible phase with the solution ofhydrolysis products. It should be noted that these procedures showcertain facts about the chemical bonding in the compound as the hydrogenfrom the triborane(7) portion of the molecule is determined separately.

The molecular structure of the compounds have been verified by X-raydiffraction analysis. The unit cell of trimethylamine triborane(7) isrhombohedral with the parameters a=6.04 A. and a=104l4'. The calculateddensity from these measurements assuming one molecule of trimethylaminetriborane(7) in the unit cell is 0.838 g./cc., which is in excellentagreement with the observed density of 0.833 g./cc. This is evidencethat empirical composition (CH) NB H-, is the molecular formula. OtherX-ray data are in agreement with the structure in which the three boronatoms form an equilateral triangle with two hydrogens bonded to eachboron atom, the seventh hydrogen atom forms a four centered bond betweenthe three boron atoms and in a plane below the boron triangle, thetrimethylamine tetrahedral group is above the plane of the borontriangle with one bond involving the two electrons of the nitrogen atomsdirected toward the center of the boron triangle. The other coordinationcompounds have likewise been shown to have a bond involving electrons ofthe group V-A or VI-A element directed toward the center of the B Hboron triangle.

The triborane(7) portion of the compound has a distinctive infra-redabsorption spectra by which it can be distinguished from the boraneadduct. Thus mixtures of triborane coordination compounds and boraneadducts, as well as mixtures of triborane(7) adduct and base, can bereadily analyzed by infra-red spectrum analysis.

The triborane(7) adducts with very strong bases are characterized by anextraordinary thermal stability and resistance to hydrolysis. Forexample, trimethylamine triborane(7), dimethylamine triborane(7) andmonomethyl triborane(7) do not appear to thermally decompose to anysignificant extent at temperatures below at least 200 C.; and pyridinetriborane( 7), picoline triborane(7), and lutidine triborane(7) do notdecompose at temperatures below about 300 C. In general the phosphinecompounds are somewhat less stable than the corresponding amines, andthe arsine compounds are less stable than the corresponding phosphinecompounds. The triborane(7) adducts with strong bases are generallysolids at lower temperatures melting about room temperature and areconsiderably less stable. They generally have a finite decompositionpressure at room temperature and decompose more rapidly above theirmelting point. The triborane(7) adducts with Weak bases are liquids oflow volatility and are less stable than the adducts with strong bases.

The triborane(7) coordination compounds are generally very soluble inpolar solvents other than water and only moderatly soluble or insolublein non-polar solvents. For example, trimethylamine triborane(7) andpyridine triborane(7) are very soluble in alcohols, e.g. methanol; verysoluble in tetrahydrofuran; moderately soluble in ethyl ether; slightlysoluble in benzene; insoluble in hydrocarbons, e.g. hexane; andinsoluble in water.

The triborane(7) adducts are excellent reducing agents for both organicand inorganic compounds. Because of their stability in alcohol solution,they are particularly useful in carrying out organic reductions inalcohol solution or reductions which give an alcohol product, such asthe reduction of acetone to isopropyl alcohol and benzaldehyde to benzylalcohol. When a methanol solution of pyridine triborane(7) was contactedwith acetone at room temperature there was a rapid reaction producingisopropanol. The use of triborane(7) coordination compounds isparticularly advantageous as no metallic element is introduced duringthe reduction processes as is the case with metal borohydrides or metaltriborohydrides. The triborane(7) adducts with very strong bases arepreferred for use as reducing agents as they are extremely stable andresistant to hydrolysis and hence are easily packaged, stored, andtransported.

The triborane adducts of strong to very weak bases are particularlyuseful in preparing alkali metal triborohydrides according to theequation where M is an alkali metal. This reaction proceeds readily atroom temperature, and if a solid triborane adduct is used it ispreferred to carry out the reaction in a solvent. For example, a mixtureof 11 mmoles ethyl ether triborane(7) and 80 mmoles of ethyl ether wascontacted in a closed reaction vessel with 10.8 mmoles solid NaH until aclear solution resulted in about 2 hours. Ethyl ether was distilled fromthe reaction mixture leaving a solid residue of NaB H The methods ofpreparing tri borohydrides from triborane(7) coordination compounds areset forth in detail in our co-pending application, Ser. No. 672,574,filed on July 18, 1957, which is a continuation-in-part application ofSer. No. 572,205, filed Mar. 19, 1956, and which has issued as US.Patent No. 3,171,712.

According to the provisions of the patent statutes, we have explainedthe principle of our invention and have described what we now considerto be its best embodiment. However, we desire to have it understoodthat, within the scope of the appended claims, the invention may bepracticed otherwise than as specifically described.

We claim:

1. That method of preparing pentaborane(9) comprising the step ofcontacting tetraborane with a substance of the group consisting of basicorganic substituted hydrides of an element selected from group V-A andbasic completely organic substituted hydrides of an element selectedfrom group VI-A, triborane(7) adducts with said hydrides, borane adductswith said hydrides and mixtures theretof, at a temperature between and60 C., and recovering the pentaborane(9) formed.

2. A method according to claim 1 in which the substance is an ether.

3. A method according to claim 2 in which the ether is ethyl ether.

4. A method according to claim 2 in which the ether is a cyclic etherand more than 1 mole of tetraborane is used for each mole of ether.

5. A method according to claim 4 in which the cyclic ether istetrahydrofuran.

6. A method according to claim 1 in which the substance is .an amine.

7. A method according to claim 6 in which the amine is a lower alkylamine and more than /2 mole of tetraborane is used for each mole ofamine.

8. A method according to claim 7 in which the lower alkyl amine istrimethylamine.

9. A method according to claim 6 in which the substance is a cyclicamine and more than /2 mole of tetraborane is used for each mole ofamine.

10. A method according to claim 9 in which the cyclic amine is pyridine.

11. A method according to claim 1 in which the sub stance is an ethertriborane(7) adduct.

12. A method according to claim 11 in which the ether is a lower dialkylether.

13. A method according to claim 11 in which the ether is a cyclic ether.

14. A method according to claim 13 in whichthe cyclic ether istetrahydrofuran.

15. A method according to claim 1 in which the substance is an aminetriborane(7) adduct.

16. A method according to claim 15 in which the amine is a lower alkylamine.

17. A method according to claim 16 in which the amine is trimethylamine.

18. A method according to claim 15 in which the amine is a cyclic amine.

19. A method according to claim 18 in which the cyclic amine ispyridine.

20- A method according to claim 1 in which the substance is a mixture ofan ether and an ether triborane(7) adduct.

21. A method according to claim 20 in which the ether is ethyl ether andthe triborane(7) adduct is ethyl ether triborane(7).

22. A method according to claim 1 in which the catalyst is a mixture ofan amine triborane(7) and an amine borane.

23. A method according to claim 22 in which the amine triborane(7)adduct is trimethylene triborane(7) and the borane adduct istrimethylamineborane.

24. That method of preparing pentaborane(9) comprising the step ofcontacting diborane with a catalyst selected from the group consistingof basic organic substituted hydrides of an element selected from groupVA and basic completely organic substituted hydrides of an elementselected from group VIA, a triborane(7) .adduct with said organicsubstituetd hydride, a borane adduct with said organic substitutedhydride, and mixtures thereof, at a temperature between 40 and 150 C.and recovering the pentaborane(9) formed.

25. A method according to claim 24 in which the catalyst concentrationis between 2 and 50 mole percent.

26. A method according to claim 25 in which the diborane pressure is inexcess of 200 p.s.i.g.

27. A method according to claim 26 in which the temperature is between60 and C.

28. A method according to claim 24 in which the pressure 300 and 122 mm.of mercury and the temperature is between and C.

29. A method according to claim 24 in which the catalyst is a mixture ofan amine borane and an amine triborane (7 30. A method according toclaim 24 in which the catalyst is a mixture of an ether and an ethertri- -borane(7).

31. That method of preparing pentaborane(9) comprising the step ofcontacting diborane with a mixture of trimethylarnine borane andtrimethylamine triborane(7) between 60 and 90 C. at a pressure in excessof 200 p.s.i.g. and recovering the pentaborane(9) formed.

32. That method of preparing penta'borane(9) comprising the step ofcontacting diborane with ethyl ether betwen 60 and 90 C. and at apressure in excess of 200 p.s.i.g., and recovering the pentaborane(9)formed.

No references cited.

EARL C. THOMAS, Primary Examiner G. PETERS, Assistant Examiner

1. THAT METHOD OF PREPARING PENTABORANE(9) COMPRISING THE STEP OFCONTACTING TETRABORANE WITH A SUBSTANCE OF THE GROUP CONSISTING OF BASICORGANIC SUBSTITUTED HYDRIDES OF AN ELEMENT SELECTED FROM GROUP V-A ANDBASIC COMPLETELY ORGANIC SUBSTITUTED HYDRIDES OF AN ELEMENT SELECTEDFROM GROUP VI-A, TRIBORANE(7) ADDUCTS WITH SAID HYDRIDES, BORANE ADDUCTSWITH SAID HYDRIDES AND MIXTURES THEREOF, AT A TEMPERATURE BETWEEN O* AND60*C., AND RECOVERING THE PENTABORANE(9) FORMED.